A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In order to be able to project ever smaller structures onto substrates, it has been proposed to use EUV radiation which is electromagnetic radiation having a wavelength within the range of 10-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm.
Radiation may be produced using plasma. The plasma may be created, for example, by directing a laser at a fuel, such as particles of a suitable material (e.g. tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector such as a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. Such a radiation source is typically termed a laser produced plasma (LPP) source.
In addition to radiation, the plasma of a plasma radiation source produces contamination in the form of particles, such as thermalized atoms, ions, nanoclusters, and/or microparticles. The contamination is output, together with the desired radiation, from the radiation source towards the radiation collector and may cause damage to the normal incidence radiation collector and/or other parts. For example, LPP sources that use tin (Sn) droplets to produce the desired EUV may generate a large amount of tin debris in the form of: atoms, ions, nanoclusters, and/or microparticles. Herebelow, reference is made to the term particulate, which means to encompass any debris or contamination in the form of atoms/ions or atom clusters from the fuel source. It is desirable to prevent the debris from reaching the radiation collector, where it may reduce EUV power, or end somewhere in the source vessel where it may create other problems. To stop especially the ions, a buffer gas can be used, but with this kind of debris mitigation, a large flow of buffer gas may be needed, which may make it desirable to have large pumps and a large supply of buffer gas. Due to the large flow of the buffer gas, the plasma region may become instable, but the flow may not stop micro-droplets of fuel from being deposited on the walls of the source vacuum chamber.
In addition, EUV LPP sources generate a large amount of fuel debris of which a part may be deposited in the central cone. The present invention is concerned with preventing build up of fuel debris deposits in the inner cone, of which uncontrolled release may damage the optics arranged in the plasma source.